Corynespora leaf spot, more commonly called cotton target leaf spot, is real and can cause problems in cotton fields if left untreated. But how much trouble it can cause is still being debated in the industry. Still, a lot was learned about this new disease in 2012.
"Bottom line is target spot is very real, and 2012 was an important year for it; it is something growers need to be aware of and prepared for (in 2013). … It's an aggressive disease and can cause significant premature defoliation and symptoms can be reduced with a fungicide application," said Bob Kemerait, University of Georgia Cooperative Extension plant pathologist.
New cotton disease
Corynespora leaf spot was unheard of before being suspected in southwest Georgia in 2005. It was positively confirmed in 2009. In 2012, Kemerait submitted evidence of the new disease to the American Phytopathological Society. Unlike other cotton leaf spot diseases, it is not caused or prompted by low fertility, most often low potassium levels.
Overall yield impact from the disease isn't known, or not enough data is in to be conclusive at this point, Kemerait said. But he feels confident that the disease can claim 100 pounds of lint per acre or more if left untreated, depending on when it first enters a field. Some anecdotal reports put the yield lose much higher.
If fungicides are recommended, what is the timing on application? That isn't certain either, but he does believe strobilurin fungicides are more effective in controlling the disease than tebuconazole-based fungicides.
Corynespora leaf spot risk
Not every acre in cotton needs to be sprayed and not every field that gets this disease sees a yield increase by using fungicides, especially if the disease doesn't hit a field until late in the year. In fact, if it hits late, it could provide some good timely defoliation to help cotton development. But then again, that's betting on an untreated disease to do your bidding. "Like I've said, if I could make this disease stop after 20% defoliation, I'd bottle it myself and sell it," Kemerait said. "But we can't predict that."
It's important to scout fields for this disease before first bloom, he said. Look for it in fields back to back in cotton, fields with a history for the disease and fields with conservation tillage practice in it. Spores can survive in the residue and splash back up to young plants. Irrigated fields and high-yielding, rank-growth cotton varieties are suspected to be at risk for the disease, he said.
Positive identification is the first step in managing this disease. One challenge for growers is that positive disease identification requires a laboratory test. But, as growers well know, assumptions and fungicide applications are expensive relative to the minimal costs to have a leaf spot properly identified.
"The bottom line is that there are no clear guidelines for control of foliar diseases in cotton," said PhytoGen cotton development specialist Russell Nuti. "We are working together to develop a better understanding of how to best manage them."
"How important it's going to be in year-to-year management, whether it's going to be an issue every year, are questions still to be answered," said PhytoGen cotton development specialist Steve Brown.
Kemerait, along with Auburn University pathologist Austin Hagan, plan to build a risk index for growers to use to gauge and manage their field's risk for the disease. It should be available in early 2013.
"What we do in land-grant research and extension is try to take the speculation out, especially in a new disease like this one. We wouldn't be doing our jobs if we didn't take this seriously and find ways to help growers manage it," Kemerait said.